The present invention relates to an automatic method of color correction for an electronic printing system and, more particularly, to a method of calibrating a color printer and a color monitor so that the appearance of a color print generated by the electronic color printing system closely approximates the appearance of the color image on the monitor screen at the time of printing.
Various color correction methods are known in the prior art for matching a scanned color original document to a printed color output. Representative disclosures are found in the following U.S. Pat. Nos.
4,929,978 (Kanamori et al.) discloses a method wherein a color scanner scans an original color image to obtain successive input color values, each value expressed in tricolor color space. A color correction table memory is responsive to one of the input color values for producing corrected data comprising corresponding output color values. A color printer responsive to the output color values reproduces the original color image as an output color image.
U.S. Pat. No. 4,500,919 (Schreiber)--FIG. 1 shows the basic system for scanning an original color document and analyzing it to obtain color patch input data values. A color correction system is used to modify the color image on a monitor and color print so that they look like the original scanned document.
There are several techniques known in the art for matching a color video display to a color print. U.S. Pat. No. 4,670,780 (McManus et al.) describes a method of matching hardcopy colors to video display color using a series of color transforming algorithms to effect the color matching. U.S. Pat. No. 4,875,032 (McManus et al.) discloses a method for reproducing colorimetric parameters of a color sample for reproducing the color of the sample on a video monitor screen. A color filter assembly and an associated sensor are used to generate signals indicative of the colorimetric parameters of the illuminated color sample. A computer transforms the signals into a form suitable for driving a video monitor to reproduce the color sample on the monitor screen. Also disclosed is a partially automated system for calibrating the intensity response of CRT monitor phosphor sets to each of a plurality of electron gun control levels utilizing the color filter assembly and associated sensor.
For some electronic color printing systems, it is more important that the color print produced by the color printer matches the color image displayed on the color monitor. This mode of operation is sometimes referred to as "What you see is what you get" (WYSIWYG). One example would be a system where a color image is created or present in computer software, and therefore does not exist as an original color document which must be scanned and inputted to the computer. Another situation would be a mixed, computer-created image consisting of stored color signals merged with inputted signals from a scanned document, or inputted digital color signals from a scanned color photograph. In these situations the operator composes the image on the video screen by manipulating the electronic images, varying the colors on the screen, using readily available software, and performing other editing functions such as cropping, air brushing, merging, etc. When he is satisfied with the image on the screen, the present invention describes a technique for producing a color print that matches as close as possible what is displayed on the screen. Thus, the color print is matched to the video display.
The present invention is directed towards a novel calibration process in which an input scanner is used to calibrate the printer, (although other suitable colorimetric measuring devices can also be used) and a monitor calibration device is used to calibrate the monitor. For the printer calibration, a number of test pattern patches are printed, under control of calibration software in a computer, which span the entire color gamut which the printer is capable of printing. The printed test patterns are scanned by an input scanner (or other suitable colorimetric measuring device), adapted to simulate the CIE tristimulus functions. The computer software analyzes the scanned readings and tristimulus values [Xp, Yp, Zp] are established for each patch. These coefficients [Xp, Yp, Zp] are associated with the corresponding [C, M, Y, K] print signals which were used to print that patch.
The monitor is calibrated by means of an arrangement including three photodiodes, each with an associated color filter, to simulate the tristimulus response. A lens may or may not be used. This calibration technique includes ambient room lighting conditions and therefore views the monitor screen in a close approximation to how the monitor image would appear to an operator. Under control of the monitor calibration software, the colors of the entire monitor screen are sequentially changed, bringing up all possible color values that the computer is capable of generating and displaying. Each color reading is displayed with specific values [Bm, Gm, Rm] assigned to the blue, green, and red electron guns in the monitor. The trichromatic coefficients [Xm, Ym, Zm] for each monitor color are read by the monitor calibration device. These coefficients [Xm, Ym, Zm] are associated with the corresponding electron gun codes [Bm, Gm, Rm]. The calibration process, described in greater detail below, results in a mapping of the color monitor electron gun generation codes [Bm, Gm, Rm] to the color actually measured by the calibrating device [Xm, Ym, Zm], which is a close approximation to what the operator's eye would see on the screen.
In operation, following printer and monitor calibration, the software limits the operator's choice of colors to fall within the range of which the printer is capable of creating.
Thus, the present invention is generally directed towards a method for calibrating a color printer and a color monitor so that a computer generated print code [C, M, Y, K] is sent to the printer which causes a print to be made which has the same tristimulus values [Xp, Yp, Zp] as the color that appears on the monitor [Xm, Ym, Zm]. More particularly, the present invention relates to a method for calibrating a color printing system which includes a color monitor, color printer, and source of digital color signals including the steps of:
storing in a controller memory, a predetermined number of sets of C, M, Y, K print codes, each set associated with a specific color value to be printed,
applying said print code sequentially to a color printer for producing color output prints carrying color patches, each patch corresponding to a specific C, M, Y, K print code applied to said printer,
measuring each said color patch under a particular set of spectral illumination conditions and establishing tristimulus values Xp, Yp, Zp, for each measured color patch,
correlating each measured Xp, Yp, Zp value with the associated C, M, Y, K print code, and storing each correlation in a memory lookup table,
storing in said controller memory, a predetermined number of sets of coded digital signals Bm, Gm, Rm, representative of all possible colors capable of being created on a particular color CRT monitor,
applying said coded digital signals sequentially to the associated electron gun of said color CRT monitor, each coded digital signal set creating a color image with a specific Bm, Gm, Rm, tristimulus value,
measuring each said color image in an environment which closely approximates the actual viewing condition of an operator observing the color image at a specific distance from the monitor, each measurement establishing a set of tristimulus values, Xm, Ym, Zm, and
correlating each Xm, Ym, Zm set with the associated applied Bm, Gm, Rm gun code signals, and storing each said correlation in said memory lookup table whereby upon selection by an operator of a print operation to print a specific color image displayed on the monitor, a print code C, M, Y, K is generated which is correlated with the stored monitor gun code that produced that particular color image on the screen resulting in an output print that has the same tristimulus values as the color image on the screen.